1. Field of the Invention
The present invention relates to a sound projection technology for projecting a sound. More particularly, the present invention relates to a projection technology for projecting a low-frequency sound.
2. Description of the Related Art
A propagation-loss of the low-frequency sound is less than that of the high-frequency sound underwater. And, a reaching distance of the low-frequency sound is more than that of the high-frequency sound. Therefore, the low-frequency sound is useful for a sound source buoy, a sonar, etc. While a frequency band, referred to as low-frequency, is not defined strictly by experts, it ranges roughly from hundreds Hz to a few KHz in a sector of a sonar system associated with the present invention. A frequency, as is more than 10 KHz, is referred to as a medium frequency or a high frequency.
An underwater projector which can project the low-frequency sound underwater is disclosed by, for example, Japan Patent Laid-Open No. 10-126877 (literature 1) and Japan Patent No. 2985509 (literature 2). The literature 1 discloses an underwater projector by a water column resonance method. This underwater projector projects a sound by causing a medium (water column) inside a cylindrical resonator to resonate.
And, the literature 2 discloses an underwater projector by a bending resonance method. This underwater projector projects a sound by causing a disk-type resonator to bending-resonate.
A low-frequency projector of the literature 1 has such an excellent advantage that it can be used under a very deep water pressure. However, as a projection frequency is lower, the scale of this underwater projector is bigger. When a sound is projected by using a resonance of a water column inside the cylindrical resonator, a sound resonance frequency F is expressed as follows.F=α1*C/(L+α2*R)  (1)Where C refers to a sound velocity in a medium inside the cylinder, L refers to a cylinder length, R refers to a cylinder radius. α1 and α2 are correction coefficients of a cylindrical shape.
It is apparent from this formula (1) that the cylinder length L and/or the cylinder radius R need to be larger so that the sound resonance frequency F can be lower.
And, the underwater projector of the water column resonance method has also such a difficult point that the projection frequency changes depending on a water depth at which the underwater projector is used. This is because the sound velocity in the medium inside the cylinder changes depending on a water depth. This may be also apparent from the formula (1).
This problem can be solved by installing a pressure compensator in the underwater projector, which expands and contracts according to the increase and the decrease of the medium pressure. However, because the installation of the pressure compensator causes an axis length of the underwater projector to be longer, the scale of the underwater projector is larger.
On the other hand, the underwater projector of the literature 2 projects a sound by causing the disk-type resonator to bending-resonate. While this disk-type resonator projects large amplitude of sound, it is small. Thus, considering this point, the underwater projector of the literature 2 is suitable for the underwater projector which projects the low-frequency sound. And, an output sound frequency of this underwater projector does not depend on a water depth.
Next, the underwater projector of the literature 2 will be described according to FIG. 1 and FIG. 2. FIG. 1 illustrates a cross-section oblique perspective view of this underwater projector, and FIG. 2 is a cross-section view illustrating 2-dimensional axial symmetry resonance mode of this underwater projector.
The underwater projector 100 of FIG. 1 provides two disk-type resonators 103. Each disk-type resonators 103 includes an active disk 101 formed from piezoelectric ceramics and a disk 102 which can bend freely as attached on one side of this active disk 101. The two disk-type resonators 103 are placed face-to-face through an o-ring 104 so that the active disk 101 is outside, and the disk 102 is inside. And, the two disk-type resonators 103 and the o-ring 104 are covered water-tightly by a mold 105.
In the underwater projector 100 configured as above, the two disk-type resonators 103 are driven by driving signals of a same frequency. The driving signals supplied to each of the two disk-type resonators 103 are in an opposite phase each other. Thus, because the two disk-type resonators 103 bending-resonate in an opposite phase each other, this underwater projector 100 projects the low-frequency sound at a high sound pressure in spite of a small scale.
However, this underwater projector 100 of the literature 2 can not be used under a very deep water pressure which is no less than a certain level. The reason is as follows.
The underwater projector 100 of FIG. 1 has an air layer inside. Thus, the water pressure at which this underwater projector 100 is usable is limited within a stress limit of the mold 105. The mold of this underwater projector 100 is stress-destructed if it is placed under a water pressure which is no less than a prescribed value.
While such problem can be solved by installing a pressure compensation mechanism inside, the installation of the pressure compensation mechanism causes the underwater projector 100 itself to be larger.